Question

What were some of the factors that allowed the Black Death to spread so quickly and with such deadly results?

Answer 1

Poor hygiene, and pest infestations and people living close to each other.
Of course, back then, the church wanted people to believe it was "DA JEWS SPREADIN THEIR SATAN PLAGUE" but that is completely false.

Answer 2

Some factors:  Interaction between communities via trade, along with poor hygiene that led to lice and fleas which carried the disease.  The fact that people didn't understand the disease and how to prevent its spread also contributed to the problem.

Context/details:

The History Channel gives this report of the coming of the Black Death to Europe from the Central Asia region:  

• The plague arrived in Europe in October 1347, when 12 ships from the Black Sea docked at the Sicilian port of Messina. People gathered on the docks were met with a horrifying surprise: Most sailors aboard the ships were dead, and those still alive were gravely ill and covered in black boils that oozed blood and pus. Sicilian authorities hastily ordered the fleet of “death ships” out of the harbor, but it was too late.

The disease spread to Europe and killed more than a third of the continent's population.   Fleas on rats that were on board the ships are usually blamed for the spread of the plague -- though new research suggests the transmission of disease-infected lice from person to person was more likely the main cause of the pandemic.

A recent paper published in the Proceedings of the National Academy of Science has cast much doubt on the idea that rats were the primary cause of the Black Death's spread. A team of researchers from the universities of Oslo and Ferrara have concluded that the spread of the disease mostly occurred due to lice and fleas on human hosts (not rats). Prof. Nils Stenseth, from the University of Oslo said: “The conclusion was very clear. The lice model fits best. ... It would be unlikely to spread as fast as it did if it was transmitted by rats.  It would have to go through this extra loop of the rats, rather than being spread from person to person.” (Quoted in January 16, 2018 article in The Independent.)

Ultimately, the spread of the plague occurred the way communicable diseases typically spread.  With any communicable disease, which is spread by communicating (moving) from one person to another, as long as there was some form of contact then the disease could spread.